1. Field of the Invention
This invention relates generally to devices known as "power tongs" which grip and rotate tubular members, such as drill pipe. More particularly, this invention relates to ring gear camming members contained within power tongs, which urge gripping jaws into contact with pipes.
2. Prior Art
Power tongs have been in existence for many years, and are generally employed in the oil and gas industry to grip and rotate tubular members, such as drill pipe. It is necessary to grip drill pipe with high compressive forces while applying a high degree of torque in order to break apart or tighten threaded pipe connections. In most cases, power tong designs employ a cam mechanism for converting a portion of the torque into a gripping (compressive) force normal to the pipe. This conversion is often accomplished utilizing a power driven ring gear having an interior cam surface. A cam follower (roller) on a jaw member rides upon the cam surface. As the ring gear is rotated, the follower (and thus the jaw member) is urged into contact with the pipe. An example of such an arrangement can be seen in U.S. Pat. No. 4,404,876.
Most current power tong designs include a ring gear camming member with an open slot or throat, through which the drill pipe is passed in order to place the power tong in position around the pipe. Some tong designs employ a ring gear camming member which has no open throat and is thus a solid circular member. However, a power tong with a solid ring gear camming member must be employed by passing it over the end of a pipe, since there is no open throat to facilitate installation. A power tong with a solid ring gear must be left in place around the pipe until conditions permit removal by sliding the tong off one end of the pipe.
Due to the tremendous forces generated during use, open throat power tongs must resist spreading during use. Prior art open throat tongs employ heavy duty rollers and other support structure to resist spreading. Despite such precautions, prior art tong often spread and fail during use, resulting in tremendous costs and down time during expensive drilling operations. While power tongs having solid circular camming members do not have the spreading problem, the versatility of open throat designs is much preferred.
Additionally, a particular power tong can only accommodate a relatively small range of pipe diameters for effective operation due to the sensitive response characteristics of the cam surfaces of the tong. This condition makes it necessary to have two or more power tongs available in order to accommodate a variety of pipe sizes encountered in drilling. In many cases, the additional weight and space requirements of extra power tongs are burdensome on drill rigs. This is especially true for offshore drilling situations.